make the change in the grouper

Author: Marco Gorelli

pandas/core/groupby/ops.py

@@ -299,9 +299,13 @@ def reconstructed_codes(self) -> List[np.ndarray]:
     def result_index(self) -> Index:
         if not self.compressed and len(self.groupings) == 1:
             return self.groupings[0].result_index.rename(self.names[0])
-
         codes = self.reconstructed_codes
-        levels = [ping.result_index for ping in self.groupings]
+        levels = [
+            ping.result_index
+            if ping.result_index.is_monotonic
+            else algorithms.factorize(ping.result_index, sort=True)[0]
+            for ping in self.groupings
+        ]
         result = MultiIndex(
             levels=levels, codes=codes, verify_integrity=False, names=self.names
         )

pandas/core/indexes/multi.py

@@ -1644,13 +1644,7 @@ def _lexsort_depth(self) -> int:
         -------
         int
         """
-        if all(x.is_monotonic for x in self.levels):
-            # If each level is sorted, we can operate on the codes directly. GH27495
-            int64_codes = [ensure_int64(level_codes) for level_codes in self.codes]
-        else:
-            int64_codes = [
-                ensure_int64(algos.factorize(self.get_level_values(i), sort=True)[0]) for i in range(self.nlevels)
-            ]
+        int64_codes = [ensure_int64(level_codes) for level_codes in self.codes]
         for k in range(self.nlevels, 0, -1):
             if libalgos.is_lexsorted(int64_codes[:k]):
                 return k

pandas/tests/test_multilevel.py

@@ -1100,15 +1100,14 @@ def aggf(x):
             return getattr(x, op)(skipna=skipna, axis=axis)
 
         leftside = grouped.agg(aggf)
-        breakpoint()
         rightside = getattr(frame, op)(level=level, axis=axis, skipna=skipna)
         if sort:
             rightside = rightside.sort_index(level=level, axis=axis)
             frame = frame.sort_index(level=level, axis=axis)
 
         # for good measure, groupby detail
         level_index = frame._get_axis(axis).levels[level].rename(level_name)
-        breakpoint()
+
         tm.assert_index_equal(leftside._get_axis(axis), level_index)
         tm.assert_index_equal(rightside._get_axis(axis), level_index)
 
@@ -2415,7 +2414,6 @@ def test_sort_non_lexsorted(self):
         )
 
         df = DataFrame({"col": range(len(idx))}, index=idx, dtype="int64")
-        breakpoint()
         assert df.index.is_lexsorted() is False
         assert df.index.is_monotonic is False